There was initially concern among cyber-security researchers that the issue affected the core protocol of PGP – meaning that all uses of the encryption method, including file encryption, could be made vulnerable.

The issue had been “overblown” by the EFF, said Werner Koch, of GnuPG.

His colleague Robert Hansen said on Twitter that the issue had been known about for some time.

He argued it wasn’t really a vulnerability in the OpenPGP system but rather in email programs that had been designed without appropriate safeguards.

‘Real secrets’ risked

Security expert Mikko Hypponen, at F-Secure, said his understanding was that the vulnerability could in theory be used to decrypt a cache of encrypted emails sent in the past, if an attacker had access to such data.

“This is bad because the people who use PGP use it for a reason,” he told the BBC.

“People don’t use it for fun – people who use it have real secrets, like business secrets or confidential things.”

Alan Woodward, at the University of Surrey, agreed, adding: “It does have some big implications as it could lead to a channel for sneaking data off devices as well as for decrypting messages.”

The researchers have said that users of PGP email can disable HTML in their mail programs to stay safe from attacks based on the vulnerability.

It is also possible to decrypt emails with PGP decryption tools separate from email programs.